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Unusual Ionospheric Absorption Characterizing Energetic Electron Precipitation Into the South Atlantic Magnetic Anomaly

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Unusual Ionospheric Absorption Characterizing Energetic Electron Precipitation Into the South Atlantic Magnetic Anomaly Earth Planets Space, 54, 907–916, 2002 Unusual ionospheric absorption characterizing energetic electron precipitation into the South Atlantic Magnetic Anomaly Masanori Nishino1, Kazuo Makita2, Kiyofumi Yumoto3, Fabiano S. Rodrigues4, Nelson J. Schuch5, and Mangalathayil A. Abdu4 1Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 442-8507, Japan 2Basic Education Series, Physics, Takushoku University, Hachioji, Tokyo 193-8585, Japan 3Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan 4INPE, Sao Jose dos Campos, Sao Paulo 12201-970, Brazil 5INPE, Southern Space Research Center, Rio Grade de Sul 97119-900, Brazil (Received December 7, 2001; Revised September 25, 2002; Accepted September 25, 2002) An imaging riometer (IRIS) was installed newly in the southern area of Brazil in order to investigate precipitation of energetic electrons into the South Atlantic Magnetic Anomaly (SAMA). An unusual ionospheric absorption event was observed in the nighttime (∼20 h LT) near the maximum depression (Dst ∼−164 nT) and the following positive excursion during the strong geomagnetic storm on September 22–23, 1999. The unusual absorption that has short time-duration of 30–40 min shows two characteristic features: One feature is a sheet structure of the absorption appearing at the high-latitude part of the IRIS field-of-view, showing an eastward drift from the western to the eastern parts and subsequent retreat to the western part. Another feature is a meridionally elongated structure with a narrow longitudinal width (100–150 km) appearing from the zenith to the low-latitude part of the IRIS field- of-view, enhanced simultaneously with the sheet absorption, and is subsequently changed to a localized structure. These features likely characterize precipitation of energetic electrons into the SAMA ionosphere, associated with substorm occurrences during the strong geomagnetic storm. From the eastward drift (∼250 m/s) of the sheet absorption, precipitating electrons are estimated to be ∼20 keV energies, assuming plasmaspheric electric fields of 1.8 mV/m. However, no ionospheric effect due to the precipitating electrons was definitely detected by the ionosonde measurements at Cachoeira Paulista, separated eastward by about 1000 km from the IRIS station. 1. Introduction (22.7◦S, 45.0◦W), Brazil showed frequent nighttime occur- It is generally understood that quasi-trapped particles in rences of high blanketing frequencies ( fb Es). They inter- the inner radiation belt sink to the South Atlantic Magnetic preted as a regular nighttime ionization source in the SAMA. Anomaly (SAMA) which is characterized by a global min- Batista and Abdu (1977) exhibited significant enhancements imum in the Earth’s total magnetic field intensity. Possible in the sporadic-E layer parameters over the geomagnetic aeronomic effects by particle precipitation into the SAMA anomaly for short periods (2–3 hours) following moderate region were reviewed so far by several literatures (e.g., magnetic disturbances. Paulikas, 1975; Gledhill, 1976: Pinto and Gonzalez, 1989). On the balloon-borne experiments in the SAMA, Pinto On the ground-based observations, ionization effects in- and Gonzalez (1986) observed intensification of X-ray fluxes duced by precipitation of energetic electrons were inves- with energies between 30 keV and 150 keV in the atmo- tigated by broad-beam 30 MHz riometers and very-low- sphere during a strong geomagnetic storm using an omni- frequency (VLF) techniques. The riometer absorption at directional scintillation detector. They considered as the Atibaia in Brazil indicated an eastward drift of about 100 first evidence on the precipitation associated with the geo- m/s, associated with a sudden commencement followed by a magnetic activity inside the anomaly region. Jayanthi et al. strong geomagnetic storm (Abdu et al., 1973). Phase record- (1997) observed count rate increases of X-rays in the energy ings of 13.6 kHz Argentina VLF signals at Atibaia in Brazil range of 15 keV to 120 keV during a mild storm on a strato- showed significant perturbations due to the lowering of the spheric balloon experiment at L = 1.3 in the SAMA. VLF reflection level, associated with the onset of geomag- Using the Injun 3 satellite measurements of 40 keV pre- netic disturbances (Abdu et al., 1981). Phase advances of the cipitating electrons, Torr et al. (1975) have theoretically es- 13.6 kHz Argentina VLF signals were linearly proportional timated that fluxes of precipitating electrons on L = 2in to a logarithmic function of the intensity of the geomagnetic the SAMA reach ∼35 times greater than the average flux on storm, given by the peak Dst (Pinto et al., 1990). Abdu and L = 2 in the northern hemisphere. From a four-year sur- Batista (1977) revealed from the ionosonde measurements vey by the Atmospheric Explorer Satellite (AE-C) over the that sporadic E-layer enhancements over Cachoeira Paulista SAMA, Gledhill and Hoffman (1981) demonstrated down- ward electron fluxes with low-energy (0.2–26.1 keV) for Copy rightc The Society of Geomagnetism and Earth, Planetary and Space Sciences 3 < K p < 6 in nighttime over the SAMA. Vampola and (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; Gorney (1983) calculated energy deposition profiles from The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. spectra of locally precipitating 36 keV to 317 keV electrons 907 908 M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA Brazil, where the total intensity of the geomagnetic field is about 22,855 nT in 2000 (Private communication, Trivedi). In this initial paper we present an ionospheric absorption event observed by the imaging riometer during the strong geomagnetic storm on September 22–23, 1999, and discuss primarily whether the absorption feature is characterized by energetic electron precipitation into the SAMA. 2. Imaging Riometer An imaging riometer for ionospheric study (IRIS) which is capable of measuring shape, spatial scale and motion of ionospheric absorption was first developed at South Pole Sta- tion in the Antarctica in order to study dynamics of auroral particle precipitation in the high-latitude region (Detrick and Rosenberg, 1990). At present, multiple IRISs have been con- tinuously operated in the arctic region and in the Antarctica (e.g., Stauning et al., 1992; Nishino et al., 1993). Most of these IRISs adopt the antenna system consisting of a two- dimensional array with 8 × 8 half-wavelength dipoles with Butler-matrix phasing circuits at 38.2 MHz or 30 MHz, by which cosmic radio noises can be detected by an angular res- olution of half-power beam-width of 11◦. The fastest time- Fig. 1. The projection of half-power beam-width of two-dimensional 4 × 4 resolution of IRIS two-dimensional data is 1 second. Quiet beams to the ionospheric layer of 100 km altitude. day curves (QDCs) necessary to reduce ionospheric absorp- tion images from background cosmic radio noise are pro- cessed by a careful selection among about 10-days cosmic noise data on magnetically quiet days. Absorption intensities observed by the S3-2 satellite, and predicted that precip- are reduced by subtraction between decreased cosmic radio itating electrons into the SAMA could produce ionization noise and the QDCs. The QDCs are usually processed as the more than one order of magnitude greater than that expected monthly background. from scattered H Lymann α radiation at night. Kohno et al. The IRIS newly installed at the INPE-SSO consists of a (1990) obtained global distributions of energetic electrons two-dimensional array with 4 × 4 half-wavelength dipoles (0.19–3.2 MeV) from particle telescopes onboard the satel- at the operating frequency of 38.2 MHz. The separation be- lite OHZORA, in which electron fluxes were enhanced in the tween the dipoles is a half-wavelength. The two-dimensional altitude range of 700–850 km over the SAMA. array is aligned in the geographic north-south (N-S) and east- As described above, although there are many reports from west (E-W) directions, respectively. By the connection of the the ground-based, balloon-borne and satellite observations antenna array with Butler-matrix phasing circuits, the two- concerning particle precipitation into the SAMA region, dimensional 4 × 4 beams with a half-power beam-width of there remain still major uncertainties in understanding of dy- 22◦ are directed upward to the ionospheric area centered at namics in the inner radiation belt and precipitation processes the zenith. The projection of the two-dimensional 4× 4 half- of energetic electrons into the SAMA, as indicated by the re- power beams to the ionospheric layer of 100 km altitude is view paper of Pinto and Gonzalez (1989); (1) temporal and shown in Fig. 1. The field-of-view (FOV) of the IRIS is ap- spatial precipitation changes from magnetically quiet to dis- proximately 330 km in the N-S and E-W cross-sections near turbed periods (2) the role of wave-induced precipitation pro- the zenith. cesses. The recording system of two-dimensional 4 × 4 cosmic Recently, Badhwar (1997) has revealed that the center radio noise data using a personal computer (PC) is basically of the SAMA drifting westward is located at ∼30◦S and the same performance with the Ny Alesund˚ (NYA) IRIS ∼45◦W off the southeastern coastline of Brazil using does system (Sato et al., 1992). The two-dimensional 4 × 4 data rate data in June 1995 from the Skylab and Mir orbital sta- are first converted to two-dimensional 8 × 8 ones by the tions. The total intensity of the geomagnetic field is near the use of interpolation and extrapolation calculations, and are global minimum of 23,008 nT according to the International led to the NYA IRIS procession system. Time variations of Geomagnetic Reference Field (IGRF) for 1990 (Takeda et 38 MHz radio noise and absorption intensities reduced from al., 1999).
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